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Foro Internacional: Los volcanes y su impacto Arequipa, Perú, 2018 Cronoestratigrafía del volcanismo con énfasis en ignimbritas desde hace 25 Ma en el SO del Perú – Implicaciones para la evolución de los Andes centrales Jean-Claude Thouret1, Brian Jicha2, Jean-Louis Paquette1, con la colaboración de la DGR, INGEMMET3 1 Université Clermont-Auvergne, CNRS OPGC et IRD, Laboratoire Magmas et Volcans, F-63000 Clermont-Ferrand, France – [email protected] 2 Department of Geoscience, University of Wisconsin-Madison, Madison WI 53706 USA 3 DGR, Ingemmet, Dirección de Geología Regional, Av. Canadá, San Borja, Lima, Perú Palabras clave: estratigrafía, cronología, volcanismo, ignimbritas, Central Andes. Resumen corto volume ignimbrites decreased to 0.85 Myr (Thouret El sur del Perú representa el segundo campo et al., 2016, 2017). ignimbrítico de los Andes con un área que sobrepasa The 40Ar/39Ar geochronology was determined using los 25 000 km2 y volúmenes de casi 5000 km3. Se pumice from 54 ignimbrites and crystals from 22 lava prresenta la extensión, la estratigrafia y la cronología flows at WiscAr Lab in Madison (Wisconsin, USA). de 12 ignimbritas que afloran en el área de los U–Pb geochronology was determined by laser cañones profundos de los Ríos Ocoña–Cotahuasi– ablation inductively coupled plasma mass Marán y Colca (OCMC). La cronología de las spectrometry using 280 U/Pb analyses of zircons ignimbritas a lo largo de los últimos 25 Myr está from 10 ignimbrite units at LMV in Clermont- basada en 74 dataciones 40Ar/39Ar and U/Pb. Antes de Ferrand (France). 9 Ma, ocho ignimbritas con gran volumen fueron producidas cada 2.4 Myr. Después de 9 Ma, el periodo de reposo entre cada ignimbrita de volumen pequeño a moderado ha disminuido hasta 0.85 Myr. Esta cronología de las ignimbritas y de las lavas del Neogeno y Cuaternario ayuda a revisar la nomenclatura de las formaciones volcánicas utilizadas para la Carta Geológica Nacional. Además las unidades volcánicas identificadas son herramientas para reconstruir la evolución geológica del flanco occidental de los Andes Centrales durante su levantamiento desde hace 25 Ma. Junto con la cronoestratigrafia de estas unidades, datos geomorfológicos obtenidos en las cuencas y sobre otros depósitos de los cañones OCMC ayudan a precisar la historia de la incisión del flanco occidental de los Andes Centrales desde hace 25 Ma. Finalmente la cronología de depósitos de avalancha de escombros y de terrazas rocosas basada en cosmogénicos (Be10) permite precisar la evolución de los cañones durante el Pleistoceno y el Holoceno. Introduction With an area exceeding 25,000 km2 and volumes c. 5000 km3, south Peru hosts the Andes’ second largest Neogene ignimbrite field. We document the extent, Fig. 1A – Two geological maps, (a) north and (b) south of the stratigraphy and chronology of 12 ignimbrite sheets OCM region, draped on the NASA SRTM DEM, highlighting in the Río Ocoña–Cotahuasi–Marán (OCM) and the 12 ignimbrites and PDC deposits together with the Late 40 39 Miocene to Pleistocene lava flows. The legend for the Colca deep canyons (Fig. 1). Based on 74 Ar/ Ar geological maps is shown in (c) Fig. 1B. Non-volcanic and U/Pb age determinations, ignimbrite-forming bedrock has been simplified. Sites of principal logs and dated episodes span 25 Myr. Prior to 9 Ma, eight large- samples are indicated. (d) OCM ignimbrites in a total alkali v. volume ignimbrites were produced every 2.4 Myr. silica diagram. After 9 Ma, average lulls between small- to moderate- -36- Thouret et al. Fig. 1B – South of the OCM region. See caption above. 1. Extent, stratigraphy and chronology Two geochronological datasets have been used to The OCM chronostratigraphy and correlations constraint the chronology and stratigraphy of the suggest that 12 ignimbrite sheets and PDC deposits OCM Neogene and Quaternary ignimbrites. There (Pyroclastic Density Currents) have erupted every 1.9 40 39 are no discrepancies between Ar– Ar or U/Pb dates Myr on average over the past c. 25 Myr: Nazca 1, for the same ignimbrite unit, and there are no age Nazca 2, Alpabamba, Majes, Chuquibamba, discrepancies between chronology and stratigraphy. Huarcaya, Caravelí, Arma, Lower Sencca, Upper All 74 dated ignimbrites and lavas are in stratigraphic Sencca, Las Lomas and Capilla (Figs 1–2). succession within analytical uncertainty. Dates were Additional ignimbrites between c. 30 and 2.7 Ma obtained from multiple units within the majority of identified in the adjacent Orcopampa region east of the ignimbrite sheets except for the Capilla, Huarcaya the OCM region (Swanson et al. 2004) support the 40 39 and Majes ignimbrites. Published Ar/ Ar fact that pyroclastic activity became more sustained chronology on a number of ignimbrites and lava while the Cordillera uplift was taking place. No flows in the OCM region (Schildgen et al. 2007, significant (>0.6 Myr) break occurred after 5.1 Ma; 2009) are within uncertainty (within 0.1 Myr) of the instead, quasi-continuous volcanism produced three dataset presented here. generations of composite volcanoes with four We distinguish and correlate cooling units and, where intercalated ignimbrite sheets and PDC deposits, and identifiable, flow units emplaced between c. 24.43 Pleistocene monogenetic fields. and c. 0.91 Ma on the basis of stratigraphic 1. The Late Oligocene–Early Miocene (30–20 Ma) unconformities, presence of vitrophyres, changes in includes the Nazca group of ignimbrites. We lithofacies, and 40Ar/39Ar and U/Pb age determina- bracket the ‘Tacaza Group’ between 30 and c. 20 tions. Figure 2 shows how these ignimbrites fit in the Ma instead of 30–24 Ma (Mamani et al. 2010). The 30 Myr OCM stratigraphic scheme compared with upper limit of Oligocene volcaniclastic rocks the Neogene ignimbrite stratigraphy in adjacent attributed to this group has been loosely dated Orcopampa and southernmost Peru. between c. 24 or 21.7 and 15.85 Ma. The c. 30 Ma Jallua tuff (Swanson et al. 2004) east of the OCM region near Orcopampa, the oldest known -37- Foro Internacional: Los volcanes y su impacto Arequipa, Perú, 2018 ignimbrite outside the tuff beds intercalated in the 2.2 km a.s.l., was at sea level at 25 Ma as shown by Moquegua Formation, is used here as the base of marine sediment of that age at Pampa Gramadal the Tacaza Group. The uppermost limit coincides and Cuno Cuno (Cruzado and Rojas 2005). The with the base of the Alpabamba ignimbrite dated lithofacies of Nazca and Majes tuff layers in at c. 20-10 Ma (Fig. 3). Because both Nazca tuffs conglomerates suggests emplacement in shallow reflect interspersed water bodies. All younger ignimbrites such as Chuquibamba show no evidence for emplacement into water. 3. The Middle–Late Miocene (11–5 Ma) pyroclastic stage includes the 10.78 Ma Huarcaya ignimbrite, the c. 9 Ma Caravelí ignimbrite and the c. 8 Ma PDC deposits, and the oldest recognizable range of shield volcanoes (7.3–5.3 Ma). This coincides with the most recent pulse of surface uplift of the Western Cordillera (Schildgen et al. 2007, 2010; Thouret et al. 2007). The Caravelí episode was the most recent ignimbrite flare-up. After 7.3 Ma, magma output became steadier albeit smaller and contributed to the intertwined growth of volcanic ranges of composite cones with moderate volumes of ignimbrite sheets. 4. The Pliocene stage is characterized by the emplacement of the composite, multi-layered sheet of the Lower Sencca Group (5.3–2.83 Ma). The latest units of this Group were confined in deep valleys in contrast to the earliest units, which often crown the Alpabamba or Chuquibamba successions. 5. The Early Quaternary volcanic range grew after 2.27 Ma and their deposits intertwined with the Upper Fig. 2 – Composite stratigraphy of the 12 ignimbrite sheets Sencca unit at c. 1.97 Ma on average. Nevado and PDC deposits of the OCM region, lava flows and their Solimana, one of these edifices, is a probable source setting in volcaniclastic and continental siliciclastic deposits. of Upper Sencca units and Lomas PDC deposits. The Correlations are shown with other formations; in particular, with ignimbrites of Orcopampa east of the OCM region Nevado Coropuna dome cluster has grown on one of (Swanson et al. 2004), Arequipa (Paquereau-Lebti et al. the source areas of Chuquibamba and Upper Sencca 2006, 2008) and in southernmost Peru. Stratigraphy of the units. Moquegua Formation after Sempere et al. (2014). 6. The Pleistocene volcanic range <1.3 Ma was contemporaneous with, and followed the 1.56–1.26 explosive activity increasing before (Nazca 1, c. Ma ‘Lomas’ PDC deposits. The latter is slightly 24–25 Ma) and after (Nazca 2, c. 22–23 Ma) the top younger than the Arequipa Airport ignimbrite (c. 1.65 of Tacaza Group, we extend its boundary to 20 Ma. Ma) and contemporaneous with subsequent PDC 2. The Early–Middle Miocene stage (20–13 Ma) is deposits near Arequipa. The most recent ignimbrites bracketed by the Alpabamba composite sheet at of sizeable volume (Capilla in the north OCM region, the base and the Chuquibamba compound sheet at Yura tuffs near Arequipa) erupted between 1.1 and the top. We bracket the ‘Huaylillas’ Formation 0.9 Ma. between c. 20 and c. 13 Ma (instead of 24–10 Ma; 7. Finally, the Middle to Late Pleistocene Mamani et al. 2010) on the basis of the two most monogenetic fields of andesite and basaltic andesite extensive and voluminous ignimbrites in south lava flows and cones grew together with the most Peru (Alpabamba c. 20.2–18.23 Ma and recent, large composite cones at <0.6 Ma, including Chuquibamba c. 14.3–13.2 Ma), and because the the potentially active Sara Sara composite cone and Chuquibamba ignimbrite pulse preceded the Late Coropuna dome cluster and eroded by glaciers and Miocene uplift and deep river incision (Schildgen landslides.

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